Known Surface Acoustic Wave (“SAW”) devices often utilize input and output interdigital transducers (IDTs) spaced apart from each other on the surface of a solid with piezoelectric properties. In one example, the solid comprises a single crystal of a piezoelectric material, such as quartz. In another example, the solid comprises one or more thin films, some which are piezoelectric, deposited on a non-piezoelectric single crystal substrate. The input IDT converts an input electrical signal to a surface acoustic wave in the SAW device. The surface acoustic wave propagates along the surface of the solid to the output IDT. The output IDT converts the surface acoustic wave to an output electrical signal. The propagation path along the surface of the solid serves as a delay path for the surface acoustic wave. It takes a certain amount of time for the surface acoustic wave to travel from the input IDT to the output IDT. The amount of time is dependent on the material properties of the solid and the acoustic path length between the input and output IDTs.
The center frequency of a SAW device is based on the geometry of the IDTs. After the solid and IDTs have been formed and integrated into a SAW device, the center frequency of the SAW device is fixed. To produce a SAW device with a different center frequency, a designer could either select a different solid material configuration or a different IDT geometry for the SAW device. In one example, the designer could select a piezoelectric crystal with a different acoustic velocity to change the center frequency of the SAW device. In another example, the designer could change the geometry of the IDTs on the piezoelectric layer to change the center frequency of the SAW device. As one shortcoming, the center frequency of such SAW devices is fixed at fabrication. The ability to frequency tune SAW devices in prior attempts has been limited.
Thus, a need exists for improved control of the acoustic velocity of a surface acoustic wave on the surface of the solid in a SAW device.